FINDING THE TIME OF LOCAL NOON
Sundials measure time as it is. Watches measure time as we would like it to be, because it is both easier and more convenient.
Sundials are based on the idea of measuring time before and after noon. Noon is when the sun is at its highest in the sky. People tend to assume that the time between noon one day and noon the next is exactly 24 hours. In fact, the length of a day varies slightly throughout the year. The shortest days are some 23 hours, 59 minutes and 40 seconds long and occur around the 15th of September, while the longest days around Christmas are some 24 hours and 30 seconds long. The reasons for these variations are complex, and are explained in detail on our companion website, Sundials on the Internet, on the Equation of Time page.
Watches are based on the convenient assumption that all days in the year are exactly 24 hours long. Sundials take the days as they are, varying in length from 24 hours and 30 seconds in late December and 23 hours 59 minutes and 40 seconds on 15 September as stated above.
1. Find out the longitude of the place where the sundial will be installed You can do this from any good atlas - longitude lines run from north to south on the atlas. Many atlases give the latitude and longitude against the place names in the index. The latitude and longitude obtained in this way will not be the exact latitude and longitude of your sundial, although it will usually be good enough for these purposes unless you live in a large city. For a more exact method, please refer to our page on Finding your Latitude and Longitude.
2. Find out the longitude of your standard meridian The world is divided into time zones 15 deg. apart, measured from longitude 0 in Greenwich, England. Thus, the standard meridian for the United Kingdom, Ireland and Portugal is the prime meridian of 0 deg, while the rest of continental Europe keeps European Time, for which the standard meridian is 15 deg. E of Greenwich (which passes through Prague, the capital of the Czech Republic).
The time zones of North America are:
Zone Prime meridian deg. W. City near longitude Hours earlier than Greenwich
Atlantic 60° Glace Bay NS 4
Eastern 75° Philadelphia PA 5
Central 90° Memphis TN 6
Mountain 105° Santa Fe, NM 7
Pacific 120° Fresno CA 8
3. Use the Spot-On Sundials Solar Noon Calculator. This will print out a table showing the exact time of Solar Noon for each day of the year, which you will need to set up your Spot-On sundial accurately. It's helpful to keep, so that you can follow the variations in the Equation of Time throughout the year. (It will also help if people look at their watch and say that the sundial is "wrong"; you can explain that in fact sundials and watches are measuring slightly different concepts of time, and both do the job they are supposed to do very well.) If you prefer to do the calculations for yourself, please refer to our page on finding the time of local noon.
6. Set your watch accurately by a radio time signal.
7. Set up your Spot-On Sundial. At the exact time of solar noon shown in the table for the day your are setting up the Spot-On sundial, rotate the sundial on the baseplate until the ray of sunlight falling between through the slot in the gnomon is exactly over the dotted noon line.
Note that the Spot-On Sundial has the hours marked in two ways. Roman numerals indicate the winter hours, and Arabic numerals indicate Daylight Saving Time.
You may find it more convenient to set the Sundial at some other time than solar noon. It is nearly as accurate to set the shadow when it is exactly on one of the hour lines.